Solid-state RF power supplies (power supplies based on RF transistors) are preferred over RF vacuum tube power supplies for driving discharges in diffusion-cooled, sealed-off, CO2 gas-discharge lasers used in material processing applications. One reason for this preference is that solid-state power supplies are typically smaller than corresponding vacuum tube supplies and do not need to be driven by potentially lethal voltages. A single mode diffusion cooled CO2 laser typically has an efficiency of about 10%. This means that a RF solid-state power supply having a 10 kilowatt (kW) output is required to provide 1.0 kW output from the laser.
A high-power RF power supply typically includes an oscillator or frequency source connected to a series of pre-amplifier stages followed by a final power amplifier stage. Each stage includes at least one power transistor. One of the most common types of RF power transistors used in an amplifier stage is a metal oxide semiconductor field-effect transistor (MOSFET). One commercially available power MOSFET is a model NXP model BLF175, form Koninklijke Philips Electronics N.V. (Philips) of Eindhoven, Holland. This MOSFET has a power gain of between about 13 and 14 dB with an output power of approximately 40 watts (W) when operated in a class C mode at a 100 megahertz (MHz) operating frequency.
The number of transistors per amplifier stage increases as the power is increasingly amplified. By way of example, a final amplifier stage of an RF power supply having 10 kW RF output, would require about 25 of the above-mentioned Philips NXP/BLF 175 MOSFETS (transistors) connected in parallel.
Manufacturers of MOSFETs do provide power combined RF MOSFETs arranged in a push-pull arrangement in a single package to a yield higher power output than that of a single MOSFET. One such MOSFET is a Philips Model BLF 278A capable of 300 W output. A number of such MOSFET packages can be connected in parallel to yield even higher output powers.
Simply stated: as the output power of the RF power supply increases, the total number of transistors increases dramatically. Consequently, limiting the number of circuit elements, such as resistors, inductors and capacitors, per stage of amplification becomes increasingly important with increasing power output from the standpoint of size and cost of the RF power supply.
Each amplifier stage in series thereof has an output impedance matching network for matching the impedances of that stage to a following stage in this series in order to maximize power transmission to the following stage. One example of a prior-art amplification stage is schematically illustrated by the circuit diagram 10 of FIG. 1.
Here, the impedance matching network includes an inductor L1, connected in series with the drain (D) side of a MOSFET Q1 (in n-channel configuration) plus a parallel tuning capacitor C1. The RF input to the amplified is connected to the gate G of MOSFET Q1, with source S of the MOSFET being connected to ground. The matching network takes the output impedance of the MOSFET and translates it to the, usually lower, input impedance of the transistor to which it delivers RF power. The impedance of that transistor is represented in FIG. 1 by a 50 Ohm (50Ω) resistor R1. C1 is usually adjusted consistent with the inductance of L1 to obtain 50Ω impedance. The load impedance value of 50 is a common value in practice, however, the load impedance value can have some other value, with C1 selected accordingly.
An RF coupling capacitor C2 serves to prevent DC power from being fed to the input of the following stage of amplification. The value of C2 is selected to provide a small reactance, and accordingly low loss, at the operating frequency of the amplifier. Only one MOSFET transistor is depicted in FIG. 1 for simplicity of illustration. Those skilled in the RF art will recognize that MOSFET Q1 could be substituted by a higher wattage, power-combined transistor package such as the above-mentioned Philips BLF 278A, to obtain a higher output power.
An L1-C1 impedance matching network as depicted in FIG. 1 usually has transmission roll-off at frequencies beyond the operating frequency of the amplifier between about minus 6 dB and minus 10 dB per frequency octave. This serves to suppress resonances above the operating frequency of the preamplifier for stabilizing power output. Such a minus 6 dB roll-off is often inadequate for such harmonic suppression. Consequently, additional circuitry (not shown in FIG. 1) is normally added to the preamplifier for harmonic attenuation, at a cost of additional electronic components.
Each pre-amplifier in a series thereof is provided with an individual DC power input. In circuitry 10, this is represented by a DC input port VCC+ to the amplifier. It is important to isolate the RF power output of the MOSFET from the DC power supply. This is usually accomplished with a large inductor, commonly called a RF “choke” (RFC), connected in series between the DC power supply and the MOSFET. This inductor is designated L2 in FIG. 1. For additional isolation between the RF and DC, a large RF by-pass (RFBP) capacitor, C3 is also provided. Capacitor C3 is selected to have very low impedance over the RF frequency range of the amplifier. In practice several capacitors in parallel are usually required provide a low RF impedance path to ground. The combination of RFC L2 and by-pass capacitor C3 result in almost no RF energy entering the DC power supply circuitry.
DC power from the DC supply passes through inductor (RFC) L2 with only a very small loss, caused by the wire windings of the RFC. The RFC, however, presents a high impedance to the RF so only a very small amount of RF power is remaining after the RFC to enter the output port of the DC supply. To achieve a sufficiently high RF loss, the inductive reactance of the choke is usually chosen to be between about 10-times and 20-times the drain-impedance of the MOSFET. This requires that the RFC L2 be very large. Large RFCs are known to exhibit poor high frequency characteristics which can contribute to amplifier instability. At high pulse repetition frequencies (PRF), pulse ringing, with high voltage peaks, is commonly encountered in the circuit of FIG. 1. The high voltage peaks of the pulse ringing arise because of the high value of the inductance in the RFC. Such high voltage ringing is undesirable because it deteriorates the reliability of the MOSFET. Because of this, additional circuitry (not shown) is usually added to suppress such ringing.
FIG. 2 is a graph schematically illustrating computed transmission in decibels (dB) as a function of frequency in megahertz (MHz) for one example of a prior art, 40 W output preamplifier stage, constructed according to the circuit arrangement of FIG. 1, and having an operating frequency chosen as 81 MHz, indicated by circle 1. Transmission at the second harmonic (162 MHz) is indicated by circle 2. Transmission at the third harmonic (243 MHz) is indicated by circle 3. It can be seen that second and third harmonics are attenuated by approximately −9 db and −16.25 dB respectively. The attenuation at 1 MHz is only approximately −18 dB which in most cases is considered insufficient.
In summary, prior-art RF amplifier circuitry as represented schematically by circuitry 10 of FIG. 1 has a relatively shallow roll-off at harmonic frequencies which leads to parasitic oscillation in the output. The circuitry also requires a large, heavy, and expensive RF choke in combination with a bank of low impedance capacitors to isolate a DC power supply for the amplifier from RF output of the amplifier MOSFET. In addition to size, weight, and expense, the large inductance RF choke contributes to pulse ringing and instability, a problem which increases with rapid rise and fall time of RF pulses as the pulse repetition frequency (PRF) increases. There are presently CO2 laser material-processing applications that require PRFs as high as 200 kHz. To reduce or eliminate parasitic oscillation and pulse ringing in the circuit of FIG. 1 additional circuitry is usually added. This additional circuitry further increases the size, complexity, and cost of a solid-state RF power supply that uses the prior-art circuitry. There is a need for circuitry that can mitigate if not eliminate shortcomings of prior-art circuitry, in order to facilitate development of CO2 lasers having average power output of several kilowatts.